Proton exchange membrane water electrolysis (PEMWE) is an advanced and effective solution to the primary energy storage technologies. A better understanding of performance and durability of PEMWE is ...critical for the engineers and researchers to further advance this technology for its market penetration, and for the manufacturers of PEM water electrolyzers to implement quality control procedures for the production line or on-site process monitoring/diagnosis. This paper reviews the published works on performance degradations and mitigation strategies for PEMWE. Sources of degradation for individual components are introduced. With degradation causes discussed and degradation mechanisms examined, the review emphasizes on feasible strategies to mitigate the components degradation. To avoid lengthy real lifetime degradation tests and their high costs, the importance of accelerated stress tests and protocols is highlighted for various components. In the end, R&D directions are proposed to move the PEMWE technology forward to become a key element in future energy scenarios.
•The degradation mechanisms of key components for PEMWE are reviewed.•The alleviating strategies for the degradation are summarized.•The accelerated stress tests and protocols for PEMWE components are presented.•The future research trends for the PEMWE are discussed.
Summary
The all‐vanadium redox flow battery (VRFB) is emerging as a promising technology for large‐scale energy storage systems due to its scalability and flexibility, high round‐trip efficiency, ...long durability, and little environmental impact. As the degradation rate of the VRFB components is relatively low, less attention has been paid in terms of VRFB durability in comparison with studies on performance improvement and cost reduction. This paper reviews publications on performance degradation mechanisms and mitigation strategies for VRFBs in an attempt to achieve a systematic understanding of VRFB durability. Durability studies of individual VRFB components, including electrolyte, membrane, electrode, and bipolar plate, are introduced. Various degradation mechanisms at both cell and component levels are examined. Following these, applicable strategies for mitigating degradation of each component are compiled. In addition, this paper summarizes various diagnostic tools to evaluate component degradation, followed by accelerated stress tests and models for aging prediction that can help reduce the duration and cost associated with real lifetime tests. Finally, future research areas on the degradation and accelerated lifetime testing for VRFBs are proposed.
To efficiently extend the life span and reduce the cost of a vanadium redox flow battery, this paper systematically reviews major components and their durability studies. Various degradation mechanisms at both cell and component levels are examined with diagnostic tools for detecting degradation and mitigation strategies for alleviating degradation compiled. Following a brief introduction of accelerated stress tests and models for aging prediction, future research directions on the degradation and mitigation of a vanadium redox flow battery are pointed out.
The demand for lithium-ion batteries (LIBs) with high mass-specific capacities, high rate capabilities and long-term cyclabilities is driving the research and development of LIBs with nickel-rich NMC ...(LiNi
x
Mn
y
Co
1−
x
−
y
O
2
,
x
⩾
0.5
) cathodes and graphite (Li
x
C
6
) anodes. Based on this, this review will summarize recently reported and widely recognized studies of the degradation mechanisms of Ni-rich NMC cathodes and graphite anodes. And with a broad collection of proposed mechanisms on both atomic and micrometer scales, this review can supplement previous degradation studies of Ni-rich NMC batteries. In addition, this review will categorize advanced mitigation strategies for both electrodes based on different modifications in which Ni-rich NMC cathode improvement strategies involve dopants, gradient layers, surface coatings, carbon matrixes and advanced synthesis methods, whereas graphite anode improvement strategies involve surface coatings, charge/discharge protocols and electrolyte volume estimations. Electrolyte components that can facilitate the stabilization of anodic solid electrolyte interfaces are also reviewed, and trade-offs between modification techniques as well as controversies are discussed for a deeper understanding of the mitigation strategies of Ni-rich NMC/graphite LIBs. Furthermore, this review will present various physical and electrochemical diagnostic tools that are vital in the elucidation of degradation mechanisms during operation to supplement future degradation studies. Finally, this review will summarize current research focuses and propose future research directions.
Graphic Abstract
The demand for lithium-ion batteries (LIBs) with high mass specific capacities, high rate capabilities and longterm cyclabilities is driving the research and development of LIBs with nickel-rich NMC (LiNi
x
Mn
y
Co
1−
x
−
y
O
2
,
x
≥ 0.5) cathodes and graphite (Li
x
C
6
) anodes. Based on this, this review will summarize recently reported and widely recognized studies of the degradation mechanisms of Ni-rich NMC cathodes and graphite anodes. And with a broad collection of proposed mechanisms on both atomic and micrometer scales, this review can supplement previous degradation studies of Ni-rich NMC batteries. In addition, this review will categorize advanced mitigation strategies for both electrodes based on different modifications in which Ni-rich NMC cathode improvement strategies involve dopants, gradient layers, surface coatings, carbon matrixes and advanced synthesis methods, whereas graphite anode improvement strategies involve surface coatings, charge/discharge protocols and electrolyte volume estimations. Electrolyte components that can facilitate the stabilization of anodic solid-electrolyte interfaces (SEIs) are also reviewed and tradeoffs between modification techniques as well as controversies are discussed for a deeper understanding of the mitigation strategies of Ni-rich NMC/graphite LIBs. Furthermore, this review will present various physical and electrochemical diagnostic tools that are vital in the elucidation of degradation mechanisms during operation to supplement future degradation studies. Finally, this review will summarize current research focuses and propose future research directions.
► We summarize degradation for polymer electrolyte membrane fuel cells. ► We review various accelerated stress test protocols at component level, and stack and system level. ► We provide ...practitioners with a useful toolbox for durability study.
Durability is one of the major barriers to polymer electrolyte membrane fuel cells (PEMFCs) being accepted as a commercially viable product. It is therefore important to understand their degradation phenomena and analyze degradation mechanisms from the component level to the cell and stack level so that novel component materials can be developed and novel designs for cells/stacks can be achieved to mitigate insufficient fuel cell durability. It is generally impractical and costly to operate a fuel cell under its normal conditions for several thousand hours, so accelerated test methods are preferred to facilitate rapid learning about key durability issues. Based on the US Department of Energy (DOE) and US Fuel Cell Council (USFCC) accelerated test protocols, as well as degradation tests performed by researchers and published in the literature, we review degradation test protocols at both component and cell/stack levels (driving cycles), aiming to gather the available information on accelerated test methods and degradation test protocols for PEMFCs, and thereby provide practitioners with a useful toolbox to study durability issues. These protocols help prevent the prolonged test periods and high costs associated with real lifetime tests, assess the performance and durability of PEMFC components, and ensure that the generated data can be compared.
Ruthenium is a good catalyst for ammonia synthesis in the Haber–Bosch process and a promising electrocatalyst for electrochemical N2 reduction reaction (NRR). However, the NRR pathway on Ru is ...unclear because of the lack of information on reaction intermediates. Surface-enhanced infrared absorption spectroscopy combined with electrochemical measurements is employed to study the NRR mechanisms on Ru thin film. During the nitrogen reduction, the *N2H x (0 ≤ x ≤ 2) was detected with the band of N=N stretching (∼1940 cm–1) at potentials below 0.2 V in an N2-satureated HClO4 solution. The coverage of *N2H x on the Ru surface was significantly increased with the potential decreasing from 0.2 to −0.4 V. The formed *N2H x species could be oxidized at potentials higher than −0.1 V. In an N2-satureated KOH solution, no N-related infrared absorption band was observed on Ru surfaces, indicating that the adsorption of nitrogen molecules on Ru surfaces is very weak.
The gas diffusion layer (GDL), one of the essential components of the membrane electrode assembly (MEA), plays an important role in the performance of proton exchange membrane fuel cells. With ...respect to this essential component and its specifications, this work intends to examine the impact of GDL defects and their effects on cell performance for component quality control. To understand how GDL defect affects its performance and to what level the defect takes effect, ex situ characterization and in situ fuel cell testing are conducted by comparing pristine and defective GDLs. While ex situ GDL properties incorporate measurements of thickness, conductivity, and permeability under compression, in situ investigation mainly involves polarization curve and electrochemical impedance spectroscopy. Among different types of GDL defects, pinholes are targeted in this work. As such, the evaluation focuses on assessing the effects of varying numbers and sizes of pinhole defects under different relative humidities (RHs). Using the state‐of‐the‐art GDLs, an improved cell performance is observed with defective GDLs (evenly distributed 40 pinholes with a diameter of 0.58 mm) under 100% RH. Results also show that the effect of pinhole defects is sensitive to RH, as well as operating current densities.
Microscopic images of pinholes on both sides of the GDL under two magnifications.
► We summarize PEMFC performance degradation under startup and shutdown processes. ► We review various system strategies for startup and shutdown processes of PEMFCs. ► We analyze PEMFC degradation ...mechanisms under startup and shutdown processes.
Performance degradation during startup and shutdown is considered an important issue affecting the durability and lifetime of proton exchange membrane fuel cells (PEMFCs). Due to the high potentials experienced by the cathode during startup and shutdown, the conventional carbon support for the cathode catalyst is prone to oxidation by reacting with oxygen or water. This paper presents an overview of the causes and consequences of performance degradation after frequent startup–shutdown cycles. Mitigation strategies are also summarized, including the use of novel catalyst supports and the application of system strategies to prevent performance degradation in PEMFCs. It is found from the literature review that improvements in catalyst supports to prevent oxidation come at the expense of high cost, and the novel supports developed to date are not sufficient to completely prevent carbon oxidation in fuel cell engines. System strategies, including potential control and reaction gas control, have been developed and applied in fuel cell engines to alleviate or even avoid performance decay. This review aims to provide a clear understanding of the mechanisms related to degradation behaviors during the startup and shutdown processes, thereby helping fuel cell material or system developers in their efforts to prevent performance degradation and prolong the lifetime of PEMFCs.
Since proton-exchange membrane (PEM) fuel cells are at the early stages of industrialization, effective manufacturing processes need to be implemented to reduce the costs. A major component of ...manufacturing processes is the product quality control. Developing product specifications and adopting standard practices and test methods are the basic requirements for quality control. The goal of this article is to establish standard specifications and test methods for quality control of bipolar plates, the second most complex and expensive part of a fuel cell stack. This work identifies and classifies the important bipolar plate properties based on their functions in the fuel cell stack. It reviews the published work on current and past test methods for determination of bipolar plate properties, and pinpoints the gaps. The standard guides, practices and test methods potentially able to fill the gaps and provide a common ground for defining and measuring the bipolar plate properties are identified and explored. We found that the current test methods used for determining the electrical contact resistance and its stability may not be repeatable and reproducible. Adopting the standards developed to determine the resistance of static electrical connections along with those to determine environmental degradation of materials can address this critical issue.
•Fuel cells for transport applications are approaching volume manufacturing stage.•Quality control is critical for volume manufacturing.•Standardized measurement of critical properties for bipolar plates is needed.•Methods for determination of bipolar plate properties are reviewed.•Standard tests, practices and guides for property measurement are suggested.
The proton exchange membrane (PEM) is a critical component of PEM fuel cells, which is typically fabricated by casting an ionomer dispersion on a substrate and then annealing the remaining ionomer ...film after solvent evaporation. Due to the high viscosity of the ionomer dispersion, gas bubbles of various sizes may get lodged within the dispersion during the casting process, and some of these bubbles may even persist after annealing. To improve the understanding of the tolerance of PEM bubbles and their impact on the fuel cell performance and durability, we fabricated Nafion™ PEMs with intentionally infused air bubbles with different sizes and quantities for fuel cell testing and evaluation against pristine, bubble-free PEMs. The results show that bubbles embedded in the PEM significantly reduce fuel cell performance and durability. In addition, the tensile strength of PEMs with bubbles is discovered to be lower than that of pristine PEMs. A burst test reveals that the failure cross-pressure of PEMs with bubbles is one magnitude lower than for pristine PEMs, suggesting heightened sensitivity to pressure differentials in PEMs containing bubbles. Overall, the low tolerance of membrane bubbles is deemed critical for production of robust fuel cells.
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•Solution casting proton exchange membranes (PEMs) with customized air bubbles.•PEM embedded air bubbles critically reduce fuel cell performance and durability.•Membranes with bubbles are sensitive to differential pressure.•Chemical membrane durability is exponentially reduced in the presence of bubbles.
Water balance has been proven to be critical not only for the performance but also for the durability of proton exchange membrane fuel cells (PEMFCs). This paper reviews experimental investigations ...and modeling works on water transport and balance in different constituents of the membrane electrode assembly (MEA), which is the most important component determining the performance and durability of a PEMFC. Major water transport mechanisms in the membrane and porous layers of MEA are summarized and the strategies to balance water in these components are also discussed. However, the experimental water transport data for different components under varied operating conditions are still insufficient and the understanding of transport mechanisms is still limited. To obtain better water management in PEMFCs, the design of the key components requires refinements. For future investigations more attention should be paid to the fundamental understanding and systematic data of water transport in each component of the MEA under varied operating conditions.
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